Light curtains are commonly used to protect hazardous areas from intruding persons or objects. For example, a particular piece of machinery may be dangerous to humans who come too close. By mounting a protective screen of so-called light curtains at the periphery of the danger zone, the presence of an intruding object or portion of a person can be detected. Detection can be used to signal an alarm, turn off the machinery, among other functions.
Light curtains are spaced-apart light emitters and light detectors. Depending upon the application, the spaced-apart distance may range from perhaps 20 cm to perhaps 6 m or more. Absent an intruding object, the light emitting portion of the light curtain emits light that passes through empty space to be detected by the light detecting portion of the curtain. However the presence of an object will block transmission of some of the light, causing the light receiving portion to output a warning signal or command. Typically the lights are turned-on in a series sequence, one at a time. FIG. 1A depicts a piece of machinery 10 to whose front edge 20 there is mounted the light receiving unit 30 of a light curtain 40, and at whose base 50 there is mounted the light emitting unit 60 of light curtain 40, according to the prior art. As indicated in FIG. 1A, the light emitting and light receiving units are coupled to electronics 70, 80, not shown here.
The emitted light 90 traverses the region or zone 100 to be protected. If the protected zone is regarded as a plane defined by an array of light emitters and array of light detectors, each array disposed in parallel straight lines, as shown in FIG. 1A, standard prior art light curtain devices 40 may be used. However when the protected region 100 involves angles and bends, it is difficult to provide a light curtain without modifying existing components. For example, it typically becomes necessary to stack portions of the light emitters on more than one plane, and to stack portions of the light detecting units on more than one plane. For example, it is desirable to maintain light beam resolution through such angles or bends, yet this design goal is not readily met in the prior art and resolution is degraded in attempting to protect curved or angular regions.
Thus there is a need for a preferably modular articulated light curtain. Preferably such light curtain should maintain light beam spacing throughout, or give the option to intentionally alter beam spacing at desired regions. Such light curtain should permit the user to shape it into desired angles, for example a zig-zag shape, and to retain the light curtain in such configuration to accommodate differently shaped regions to be protected. As noted, light beam spacing should be preserved though the desired angles. Preferably such light curtain should provide modular light emitting units and modular light detecting units, which individual modules are on a single plane, regardless of the angular configuration. As such, one pair of a light emitting module and a light receiving module will preferably be parallel to each other but can tolerate error in planarity, for example .+-.12.degree..
The present invention provides such a light curtain.